EP A 0 108 590 discloses a method of installing optical fibre units along a previously installed conduit or duct by drag forces generated by a gaseous flow blown along the duct.
Generally speaking, it is desirable to increase the distances over which optical fibre units can be blown, as otherwise it can be necessary to install separate lengths of optical fibre unit, which then have to be spliced together. Splicing involves expense and time as it will often require the digging of holes in a pavement (sidewalk) or roadway in order to gain access to the ducting and then breaking into the ducting before the splice can be made.
Many factors affect the distance over which an optical fibre unit can be blown. Two known factors are friction between the sheath and the ducting and the build-up of static charges that tend to cause the sheath to adhere to the ducting.
EP A 0 108 590 discloses the possibility of blowing compounds in liquid or powder form along the ducting prior to, or during, installation in order to provide lubrication for the optical fibre unit and suggests powdered talc as a suitable lubricant. GB-A-2 156 837 is also concerned with optical fibre units to be installed by blown fibre installation techniques. This document discloses incorporating an adherence reducing substance in the ducting and/or the sheath of the optical fibre unit. The example given is of an extruded polyethylene conduit to which is added less than 3% by volume of a compound commercially available from BXL Plastics Ltd of Grangemouth, Stirlingshire, United Kingdom. The compound is known as PZ 146 and comprises a slip agent, an anti-block agent, an anti-static agent and an antioxidant. The slip agent and anti-static agent of PZ 146 are such that they migrate to the surface of the conduit to reduce friction and improve the dissipation of static electric charges generated during installation of the optical fibre unit. There is no specific disclosure of a particular adherence reducing substance incorporated in a sheath. The document also mentions the possibility of coating a sheath with an adherence reducing substance, but provides no disclosure of how this is done or of suitable coating materials.
Other factors that affect the distances over which optical fibre units can be blown are the weight of the unit, the difference between the outside diameter of the unit and the internal diameter of the ducting and the stiffniess of the optical fibre unit.
Hitherto, commercially available optical fibre units (2, 4 and 8 fibre units) have relied on a tight package construction to provide the rigidity necessary to permit blowing. The tight resin sheath for the fibres is typically imbedded with glass beads that serve to reduce the friction between the sheath and the ducting.
One approach to increasing the potential blowing distances of these constructions would be to reduce the overall diameter of the package. However, this would reduce the number of optical fibres that could be included in the package. An alternative approach would be to reduce the thickness of the optical fibre cable sheath. However, if the sheath thickness is reduced, the inclusion of lubricating additives or glass beads in the sheath material is problematical and conventional coating processes are unsuitable for coating a thin-walled sheath.